1. Field of the Invention
The present invention relates to a liquid-droplet ejecting apparatus.
2. Discussion of Related Art
As a liquid-droplet ejecting apparatus for ejecting liquid droplets, there is known an ink-jet printer which records images based on image data by ejecting liquid droplets from a liquid-droplet ejecting head toward a recording medium.
Such an ink-jet printer includes: an ink cartridge mounted on a body of the printer; a sub tank to which an ink stored in the ink cartridge is supplied; and an ink-jet head which prints images by ejecting the ink stored in the sub tank from a plurality of nozzles.
The ink cartridge and the sub tank are not held in constant communication with each other, but are brought into communication with each other by a device for connecting and disconnecting the sub tank and the ink cartridge to and from each other, only when the sub tank is replenished with the ink stored in the ink cartridge,
In the arrangement described above, the sub tank and the ink cartridge are disconnected from each other when printing is performed, so that images are printed with the ink in the sub tank. The sub tank and the ink cartridge are connected to each other when the amount of the ink in the sub tank becomes small, so that the sub tank is replenished with the ink in the ink cartridge.
The above-described ink-jet printer needs to be configured such that the ink stored in the sub tank does not leak through the nozzles of the ink-jet head. Because the ink in the sub tank tends to flow toward the nozzles according to gravity in particular when the sub tank is disposed above the ink-jet head, some measures need to be taken to prevent ink leakage from the nozzles.
There have been proposed some techniques to prevent the ink leakage. As one example of the techniques, an ink absorbing member is disposed in the sub tank for preventing the ink leakage from the nozzles owing to the absorbing power of the ink absorbing member.
As another example of the techniques, a space in the sub tank in which the ink is not present is always subjected to a negative pressure for thereby preventing the ink leakage from the nozzles. More specifically, the space in the sub tank is evacuated to place the space under the negative pressure, and subsequently the space is hermetically closed, whereby the space is kept under the negative pressure.
However, the volume of the hermetically closed space increases in accordance with consumption of the ink in the sub tank. Accordingly, the negative pressure in the hermetically closed space increases in accordance with the ink consumption, so that the characteristics of ink ejection from the nozzles fluctuates, causing a risk of unstable image quality.
In view of the above, Patent Document 1 discloses an ink-jet recording apparatus in which the sub tank and the ink cartridge are connected only when the sub tank is replenished with ink. The disclosed apparatus is configured such that the negative pressure in the sub tank does not increase. In the following description, the negative pressure means a pressure lower than an atmospheric pressure, and the increase of the negative pressure means that the pressure which is lower than the atmospheric pressure further decreases or reduces.
The sub tank of the disclosed ink-jet recording apparatus has a communication portion for communication with the atmosphere formed at an upper portion of a casing of the sub tank, and an orifice is formed so as to extend in a downward direction from the communication portion to the vicinity of a bottom surface of the sub tank. At a lower end of the orifice, there is formed an opening through which the interior of the sub tank communicates with the atmosphere. Since the opening is disposed in the vicinity of the bottom surface of the sub tank, the opening is generally located below the surface of the ink stored in the sub tank irrespective of the height level of the ink surface.
In the thus constructed sub tank, there is formed, at the opening, an interface of the ink and the atmosphere. The surface tension of the interface and the atmospheric pressure are balanced, thereby preventing entry of the air into the sub tank through the opening. Therefore, the space in the sub tank can be kept in the hermetically closed state. Further, the negative pressure of the space in the sub tank can be maintained by hermetically closing the space after the space has been evacuated to place the space under the negative pressure for the purpose of preventing the ink leakage.
In the arrangement described above, when the ink is consumed by a recording operation, the volume of the hermetically closed space in the sub tank is increased. According to the Boyle's law showing relationship that a product of a volume and a pressure of a gas is always constant, the pressure in the space is lowered in inverse proportion to the increase of the volume of the space, so that the negative pressure in the sub tank is increased. Due to the increase of the negative pressure, the opening is subjected to a force by which the air is drawn into the sub tank.
When the negative pressure continues to increase and finally exceeds a prescribed value (threshold), the force of drawing the air into the sub tank becomes larger than the surface tension at the opening, so that the balance between the pressure of the atmosphere and the surface tension at the opening is upset, resulting in introduction of the air into the sub tank through the opening. Consequently, the volume of the gas in the sub tank is increased, whereby the negative pressure in the sub tank is decreased according to the above-indicated Boyles law. Thus, it is possible to suppress the increase of the negative pressure in the sub tank.
Patent Document 1 JP-A-2004-9450